August 2009

In urban environments, local lights shining on your telescope can be an even bigger problem than the washed-out skies. Any stray light that finds its way into your telescope tube (or more precisely, into your eyepiece) is a contrast killer, making it even harder to see anything but the brightest objects. While there is little you can do to block general light pollution, you can keep stray light from entering your telescope with a simple tube extension that serves as a light shield.

A tube extension light shield is simply a tube that extends the front of your telescope, making it harder for off-axis light to get into your telescope’s light path. These light shields are particularly effective on Newtonian telescopes because the focuser is too close to the end of the tube on many Newts (especially on many commercial models). Dew shields (such as those commonly used on Cassegrains) also make effective light shields. You can buy dew shields/light shields from various vendors. But you can also make one pretty easily for under $5. All you need is need is some black poster board paper, white glue, and packing tape. It may sound flimsy, but it only needs to stop photons and be stiff enough keep its shape. Here is how I made one for my 10 inch (250mm) Dob …

If you peruse posts on Cloudy Nights and other forums on the web, you’ll find claims that narrowband filters don’t work well with small scopes, that you need at least 8 inches (or 10 inches, or 12 inches…) of aperture to get any benefit from them. I’ve never agreed, having used the UltraBlock to advantage with my 4.25 inch (108mm) Newtonian to see through local light pollution.

Recently, I put this hypothesis to the test with my 4.25 inch (108mm) Newtonian on a night with particularly bad haze and light pollution. It was a night on which 3rd magnitude stars were hard to see. I found that planetary nebulas which were invisible in the 4.25 inch showed up relatively clearly using the UltraBlock. So as far as I can tell, you can use the UltraBlock filter to help you see through light pollution just as effectively with small apertures as with large. Here are the details….